The present invention relates to an apparatus and concomitant method for evaluating and improving the performance of signal processing systems. More particularly, this invention relates to a method and apparatus that assesses the visibility of differences between two signal sequences.
Designers of signal processing systems, e.g., imaging systems, often assess the performance of their designs in terms of physical parameters such as contrast, resolution and/or bit-rate efficiency in compression/decompression (codec) processes. While these parameters can be easily measured, they may not be accurate gauges for evaluating performance. The reason is that end users of imaging systems are generally more concerned with the subjective visual performance such as the visibility of artifacts or distortions and in some cases, the enhancement of these image features which may reveal information such as the existence of a tumor in an image, e.g., a MRI (Magnetic Resonance Imaging) image or a CAT (Computer-Assisted Tomography) scan image.
For example, an input image can be processed using two different codec algorithms to produce two different codec images. If the measure of codec image fidelity is based purely on parameters such as performing mean squared error (MSE) calculations on both codec images without considering the psychophysical properties of human vision, the codec image with a lower MSE value may actually contain more noticeable distortions than that of a codec image with a higher MSE value.
Therefore, a need exists in the art for a method and apparatus for assessing the effects of physical parameters on the subjective performance of a signal processing system, e.g., an imaging system. Specifically, a need exists for a method and apparatus for assessing the visibility of differences between two sequences of time-varying visual images.
The present invention is a method and apparatus for assessing the visibility of differences between two input signal sequences, e.g., image sequences. The apparatus comprises a perceptual metric generator having an input signal processing section, a luminance processing section, a chrominance processing section and a perceptual metric generating section.
The input signal processing section transforms input signals into psychophysically defined quantities, e.g., luminance components and chrominance components. The luminance processing section processes the luminance components into a luminance perceptual metric, while the chrominance processing section processes the chrominance components into a chrominance perceptual metric. Finally, the perceptual metric generating section correlates the luminance perceptual metric with the chrominance perceptual metric into a unified perceptual image metric, e.g., a just-noticeable-difference (JND) map.
The JND map is produced using independent spatial and temporal channels to process the input signals. To enhance the performance of the apparatus, channels having spatio-temporal filters are used to respond to point or line flicker within the signals. Specifically, filtering is performed over multiple image fields to simulate visual response to line flicker without altering response to pure spatial or temporal signals.